Production of aluminum alloy sheet products in multi-product hot mills

ABSTRACT

An improved rolling procedure is used for forming aluminum alloy sheet product of critical quality, e.g. can body stock, in a hot rolling mill. The mill is of a type in which the rolls are changed on a routine basis at least once during a production week and a plurality of rolled products of different alloy compositions, including sheet product of critical quality, are fed through the hot rolling mill on a sequential basis. In the improved procedure, the aluminum alloy sheet product of critical quality is fed through the rolling mill within the first 72 hours of production time after the rolls have been changed. Also, the order in which different products are to be produced is selected such that products that generate sufficient heat during rolling to cause micro-annealing of the rolls are processed after the sheet product of the critical quality.

FIELD OF THE INVENTION

This invention relates to a method for improving the quality of rolled aluminum alloy sheet where sheet quality is very critical, e.g. can body stock produced in a multi-product hot rolling procedure.

BACKGROUND OF THE INVENTION

Modern metal cans as used for beverages such as soft drinks, beer and the like are commonly formed of a seamless one-piece body (which includes the bottom end and cylindrical sidewall of the can, and a top end bearing an opening). The body is produced from a blank of cold-rolled aluminum alloy sheet (can body stock) by a conventional forming technique known as drawing and ironing, which involves drawing the blank into a cup (cupping) and then passing it through a succession of dies to achieve the desired elongated cylindrical body configuration, with a sidewall of reduced thickness relative to the bottom. The sidewall may have a gauge of as little as 0.013-0.015 inch and as a consequence is very susceptible to failure if any kind of flaw or impurity exists in the can body stock. For example, the sidewall may split during the ironing process.

Can body stock is typically formed from an AA 3XXX series alloy, e.g. AA 3104 alloy, which is direct-chill cast into large ingots which are scalped and homogenized, and successively hot rolled and cold rolled to the desired final gauge.

The can body stock is typically rolled into sheet of a wide width of 60 inches or more in large centralized rolling mills. Such a rolling mill typically includes a series of roll stands which reduce the thickness of the ingot in stages by being compressed between rolls of successive stands, typically beginning with reversing rolls. These large rolling mills typically produce many products in addition to can body stock, and this necessitates the use of a flexible manufacturing procedure where different products are produced sequentially on the same mill.

In a typical operation, ingots of an aluminum alloy to be rolled are scalped and heated in a furnace to a required homogenizing temperature for several hours to ensure that the components of an alloy are uniformly distributed throughout the metallurgical structure. While still hot, the ingot is subjected to hot rolling in a number of passes using reversing or non-reversing mill stands which serve to reduce the thickness of the ingot. The slab thus formed is then fed to a tandem mill for hot finishing rolling, after which the sheet stock is coiled, air cooled and stored. The coiled sheet stock is subsequently cold rolled to final gauge.

The production of can body stock is described in numerous U.S. patents such as U.S. Pat. Nos. 4,318,755, 4,614,224, 5,356,495, 5,470,405, etc.

Ferreira and Horton, U.S. Pat. No. 5,609,053 looked at the problem of improving rolling mill efficiency when different products are sequentially being fed through a hot rolling mill. The patent provided a method for selecting setup parameters for rolling stands on a rolling mill by: (1) determining several web material classes according to similar hardness and rollability characteristics; (2) determining the thickness for web material of each class to have between rolling stands such that temperatures remain substantially constant; (3) selecting a product to produce from a particular web material; (4) determining which class includes web material of the product; (5) determining the exit thickness of the product; and (6) calculating the thickness including the entry thickness for the web material according to the class of the web material.

A method of detecting surface flaws in rolled sheet is described in Smith et al, “The Use of an Electron Microprobe Mapping Technique to Improve Aluminum Hot Plate Surface Quality”, International Symposium on Aluminum Surface Science and Technology, Manchester, U.K., May 21-25, 2000. This attempted to detect surface flaws that could affect the final product.

There has been an ongoing problem in can plants during the forming of the can bodies. Typically these have been cupper jams, tear offs in the body makers and split sidewalls during the ironing process. The historical interpretation of the defects has been linked to can plant tooling.

It was eventually determined that the problem was being caused by very small particles embedded in the surface of the can body stock, as well as very small particles (10-100 μm) embodied within the sheet. These particles often fell out during the removal of a can from the can making press and because the particles are so small, accurate analysis of the specific alloy was challenging. It was eventually determined that the particles were iron/chrome based, but the source of these particles was not evident.

Almost all rolls used in aluminum rolling mills are iron/chrome based and it was concluded that the rolling mill rolls must be the source of the problem.

The object of the present invention is to find a way of substantially preventing the transfer of small particles of iron/chrome material from the rolling mill rolls to highly critical sheet products during hot rolling.

SUMMARY OF THE INVENTION

According to the present invention, it has been found that the rolling of highly critical sheet product, such as can body stock, is highly sensitive to how the rolls were used prior to rolling the particular stock. For instance, rolling of alloys having a high magnesium content, e.g. an AA 5182 alloy, may cause elevated roll contact temperatures resulting in micro-annealing of the roll surfaces. These micro-annealed rolls were found to be a source of the very small particles embedded in the sheet products, such as can body stock. The problem, therefore, was how to avoid the small particles in the can body stock while continuing to use a rolling mill to process stocks other than can body stock, as well as can body stock, without the need to modify actual mill conditions.

Rolls in a hot rolling mill are changed approximately twice during each week of production. It has been found that the problems that were occurring during the forming of aluminum alloy can sidewalls can be greatly decreased by hot rolling the can body stock within the first 72 hours of production time after the rolls have been changed and by selecting the order in which different products are produced such that products that generate sufficient heat during rolling to cause micro-annealing on the rolls are processed after the can body stock. The first 72 hours of production time includes normal times for minor maintenance as well as times for loading different process stocks, etc. Thus, actual stock rolling time during which the rolls are in contact with the stock is within about 32 hours.

It is preferred to change the rolls about twice per week and to hot roll the can body stock within the first 48 hours of production time, with an actual contact of the rolls with the stock within about 21 hours.

Thus, stocks that can affect the quality of the can body stock are run after the can body stock. These may include, in addition to alloys having a high concentration of magnesium, such materials as roll bite refusals and other mill upsets. Products such as cathode sheet, brazing sheet and anodize quality sheet may be rolled randomly with, i.e. before or after, the can body stock during the first production period, i.e. the first 72 hours, preferably 48 hours. The alloys having a high concentration of magnesium typically contain 4 to 5% by weight or more of magnesium. Examples of these alloys include AA5182, AA5086, AA5083, etc.

Examples of cathode sheet alloys include AA1070 and AA1145, while an anodize quality sheet may, for example, be made of AA5005 alloy. The brazing sheet is typically a one- or two-sided sandwich or laminated construction formed of an AA3003 type core and a layer or layers of a 4XXX series alloy, such as AA4343, AA4045, AA4147. The 4XXX alloy typically comprises about 5 to 12% by weight of the brazing sheet.

Micro-annealing of the surface of rolls typically occurs after the rolls reach a temperature of at least 575° F. During this micro-annealing, the surface of the rolls reach sufficient temperatures to soften the roll surface, which upon rolling of subsequent products can release the softened roll surface into the aluminum sheet. Avoiding the high temperature eliminates the risk of having the roll material released into the can stock sheet.

It has been found that when the sequencing according to this invention such that the can body stock is processed at any time during a working cycle of a set of rolls, there were many occurrences of production problems in can plants using the can body stock. On the other hand, when the sequencing according to the invention is followed, production problems in can plants are almost totally eliminated.

While the invention applies particularly to the production of can body stock, it can apply equally to any thin aluminum alloy sheet product wherein the quality of the sheet product is highly critical.

EXAMPLE 1

A test was conducted on a commercial rolling mill with 120 inch rolls. The test was conducted over three days during which 434 coils aluminum alloy sheet were produced from ingots of 11 different alloys. These included 178 coils of can body stock from AA3104 aluminum alloy.

The following product sequence was used: Sequence Alloy No. of Coils 1 1100/1145 15 2 3003 59 3 3004 3 4 3104 178 5 5052 91 6 5182 17 7 5754 1 8 6061 1 9 6111 1 10 X308 22 11 29XX 14

Following the can body stock (AA3104), the remaining products were run in the best available order based on delivery date, transitions and mill conditions.

Within the above test, the rolling of the can body stock was completed within about 30 hours after changing the rolls. The can stock contained no defects of the type that cause problems in a can plant during formation of can bodies. 

1. In a process for producing aluminum alloy sheet product of critical quality in a hot rolling mill of a type wherein the rolls are changed on a routine basis at least once during a production week and a plurality of rolled products of different alloy compositions, including sheet product of critical quality, are fed through the hot rolling mill on a sequential basis, the improvement which comprises hot rolling the aluminum alloy sheet product of critical quality within the first 72 hours of production time after the rolls have been changed and selecting the order in which different products are to be produced such that products that generate sufficient heat during rolling to cause micro-annealing of the rolls are processed after the sheet product of critical quality.
 2. The process as claimed in claim 1, wherein the critical quality stock is hot rolled with the first 32 hours of actual contact between the rolls and the metal stock being rolled.
 3. The process as claimed in claim 1, wherein the sheet product of critical quality is can body stock.
 4. The process as claimed in claim 3, wherein the rolls are changed after 3 to 4 days of production.
 5. The process as claimed in claim 4, wherein the can body stock is hot rolled within the first 48 hours of production time.
 6. The process as claimed in claim 5, wherein the can body stock is hot rolled within the first 21 hours of actual contact between the rolls and the can body stock being rolled.
 7. The process as claimed in claim 3, wherein the alloys causing micro-annealing of the rolls are aluminum alloys having a high concentration of magnesium.
 8. The process as claimed in claim 7, wherein the can body stock is an AA3104 alloy.
 9. The process as claimed in claim 7, wherein the aluminum alloy having a high concentration of magnesium contains at least 4% by weight of magnesium.
 10. The process as claimed in claim 9, wherein alloys that may selectively be rolled before can body stock are selected from the group consisting of cathode sheet, brazing sheet and anodize quality sheet. 